init monlet repo with stage 0-2 (docs, contract, agent MVP)

api/README.md

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+# API Contract
+
+`openapi.yaml` is the first versioned Monlet API contract.
+
+Rules:
+
+- all public endpoints are under `/api/v1`;
+- agent-facing endpoints use bearer token auth;
+- check result ingestion is idempotent by `event_id`;
+- incident keys are stable and must not include dynamic values;
+- implementation must update this contract when public API changes.